Broadband,high gain antenna with relatively constant beamwidth



hill/311L511 fil vl-v y Ill-l l-llhlllvic 5&5 39 09 May 26, 1970 F RR sET AL 3,514,781

BROADBAND, HIGH GAIN ANTENNA WITH RELATIVELY CONSTANT BEAMWIDTH FiledDec. 5, 1967 82 0000 o gggszoo I N VE N TORS,

JOSEPH E. FERR/S 8 WILEY E. Z/MMERMAN.

A TTORNE Y8.

3,514,781 Patented May 26, 1970 United States Patent Oflice 3,514,781BROADBAND, HIGH GAINANTENNA WITH RELATIVELY CONSTANT BEAMWIDTH Joseph E.Ferris, Saline, and Wiley E. Zimmerman, Ann Arbor, Mich., assignors, bymesne assignments, to the United States of America as represented by theSecretary of the Army Filed Dec. 5, 1967, Ser. No. 688,271

Int. Cl. H01q 15/02, 15/16, 19/12 US. Cl. 343840 Claims ABSTRACT OF THEDISCLOSURE In certain applications, for example in direction findingequipment, it is necessary to have an antenna with a high gain over awide frequency range and at the same time a beamwidth which does notvary appreciably with frequency. It has hitherto been impossible toattain all of these desiderata with a single antenna and in suchapplications it has been necessary to use several antennas, eachdesigned to cover a portion of the desired wide frequency range or touse a single antenna with reduced gain. The requirement of high gain, ofthe order of 20 db with respect to an isotropic source, dictates the useof a parabolic reflector. The beamwidths of parabolic antennas, however,vary with frequency, the beamwidth in any given direction being directlyproportional to the number of wavelengths across the parabolic reflectorin the given direction. Thus, for a given solid parabolic reflector, adoubling of the applied frequency will reduce the beamwidth of 50%assuming that the aperture distribution remains the same. Statedanother? way, if it is desired to maintain a constant beamwidth, thesize of the parabolic reflector must decrease as the applied frequencygoes up.

-In accordance with the present invention, the effective reflecting areaof a parabolic reflector is caused to vary inversely with frequency.This is accomplished by providing the reflector with a plurality ofconcentric circular arrays of holes, the size of the holes of theinnermost array being the smallest, with the hole size progressivelyincreasing toward the edges of the reflector. In addition to the more orless symmetrical arrangement of holes in these circular arrays, otherrandomly arranged holes are placed between and among the holes of thearrays. These randomly arranged holes improve the antenna performance bybreaking up the currents which would otherwise flow in circular paths onthe parabolic surface between the arrays of holes. The geometry of thearrays of holes is such that at the highest frequency of interest onlythe hole-free area of the parabola within the innermost hole array iseffective in reflecting and focusing the energy from the feed. At thelowest frequency of interest the diameters of the largest of the holesis smaller than the wavelength and hence the reflectivity of the entireparabolic surface is high at such wavelength. At intermediatefrequencies, the wavelengths are such that one or more of the outerarrays of large holes will be substantially transparent ornon-reflective and one or more of the inner arrays of smaller holes willbe reflective and hence the effective reflecting area of the reflectoris made frequency dependent in the desired manner.

It is thus an object of this invention to provide an improved parabolicantenna of wide bandwidth and of reduoed beamwidth variation withfrequency.

Another object of this invention is to provide a parabolic reflector,the effective reflecting area of which is large at a given frequency anddecreases at frequencies higher than said'given frequency.

These and other objects and advantages of the invention will becomeapparent from the following detailed description and drawing, the solefigure of which is an illustrative antenna embodying the novel conceptsof the present invention.

The drawing is a pictorial view looking into the aperture of an antennacomprising a parabolic reflector 7 and an offset feed 3.,The inventionis illustrated in connection with an offset feed antenna however thepractice of the invention is not limited thereto. Such antennas haveadvantages well known in the prior art, for example, the offset feed isnot in the path of the reflected energy so that there is no apertureblocking by the feed and no reradiation into the feed, which causesstanding waves therein. The illustrated antenna was designed foroperation over the frequency range of 1 to 10 gigahertz (1000 to 10,000megacycles). In order to accommodate this frequency range, it wasnecessary to use a wideband feed. The illustrated wideband feed 3comprises a ridged horn which is energized or excited by a coaxial line(not shown) which is applied to the connector 5 in the throat of theridged horn. The position of the ridge within the horn is indicated at5. Such wideband feeds per se are known and no further discussionthereof is necessary. The feed is located at the focus of the parabolicreflector, in accordance with the usual practice. The reflector 7 in theareas between the holes therein is of reflective material, for example,it may be formed of a solid metal of good reflective characteristics, orformed of fiberglass with metallic paint on the surface thereof, and theholes then drilled to form the perforated surface shown. The point 21 onthe surface of the reflector is the center of six concentric arrays ofholes, these arrays being numbered 19, 17, 15, 13, 11 and 9 from theinnermost to the outermost. As can be seen, the holes of each array arethe same size, but the hole size increases with the diameter of thearrays. The feed is arranged so that the center of its radiation patternstrikes the point 21. The diameter of the innermost array is chosen sothat the hole-free area within this innermost array has the properdimensions to achieve the desired beamwidth at the highest frequency ofoperation. Thus the hole-free area should have a diameter of severaltimes the wavelength at the highest operating frequency. The hole sizeof the inner array 19 is made approximately equal to the wavelength atthe highest operating frequency, and hence this array and all of theother arrays will be substantially transparent or non-reflective at theshortest wavelength. Thus at the highest frequency the effective size ofthe reflector is equal to the area within the innermost array. The holediameter of the outermost array is made approximately one half of thewavelength of the lowest frequency of operation and thus the entirereflector has substantial reflectivity at this frequency. As thefrequency increases the apparent or effective reflecting area goes down,for example, as the wavelength approaches the hole diameter of theoutermost array 9, most of the energy directed to this region will passthrough the large holes and will not be reflected or focused in thedirection of the antenna aperture. At a still higher frequency, both ofthe outer arrays 9 and 11 will be transparent to the radiation from thefeed 5 and the effective reflecting area will be still further reduced.

An antenna comprising only the systematic, concentric arrays of holes19, 17, 15, 13, 11 and 9 was built and tested. The radiation patterntests on this antenna were unsatisfactory in that high side lobes wereproduced and the 3 main lobe was poorly shaped. It was concluded thatthis poor radiation pattern was caused by currents flowing in thecircular metallic areas between the circular hole arrays. In order tominimize these loop currents, small, randomly spaced holes were placedon the reflector surface between the concentric arrays and outside ofthe outermost array. The effect of these randomly spaced holes was tocause the reflector surface currents to add in random fashion in the farfield of the antenna. The random distribution of the currents tended tosmooth out the undesired lobing structures noted above. The randomlylocated holes are indicated at 25 and 27in the drawing. Within theconcentric arrays of holes the random holes 27 are made smaller in sizethan the holes of the innermost array 19. Outside of the outer array 9,the random hole size can be made somewhat larger, as indicated by theholes 25. Some of the circular holes in the drawing appear to beelliptical because of the curvature of the reflector. While all of theholes in the illustrative embodiment are circular, circular holes arenot necessary to the practice of the invention.

In an antenna constructed according to the present invention, theparabolic focal length F was 13 inches and the aperture dimension D was48 inches, yielding an F to D ratio of .27. The innermost array 19 wascomposed of holes 1 inch in diameter, the next array 17 had holes of 1.4inches diameter; array 15, 1.95 inches in diameter; array 13, 2.7 inchesdiameter; array 11, 3.8 inches diameter; and outermost array 9 had holes5.3 inches in diameter. The diameter of the hole-free area around thepoint 21 was approximately 6 inches and the diameter of the circleconnected the centers of the holes of the outer array 9 wasapproximately 40 inches. This antenna exhibited a beamwidth variation of2 /2 to 1 over the frequency range of 1 to 10 gigahertz. While this isnot a constant beamwidth, it is much better than the variation whichwould be obtained in the absence of a reflector perforated in this way.A solid reflector antenna similar to that in the drawing was found tohave a beamwidth variation of more than 4 to 1 over the same frequencyrange.

Parabolic reflectors have been constructed in the past either with holesor of metallic mesh, to reduce both the weight and the wind resistancethereof, however in these prior art reflectors the holes or meshopenings have been purposely made small enough relative to the operatingwavelengths, so that the entire structure has high reflectivity over theentire frequency range of interest.

While the invention has been described in connection with anillustrative embodiment, obvious modifications thereof are possiblewithout departing from the inventive concepts disclosed herein,accordingly, the invention should be limited only by the scope of theappended claims.

What is claimed is: 1. .A. broadband, high gain antenna. with arelatively constant beamwidth comprising; a parabolic reflector, abroadbanded feed at the focus of said reflector, the surface of saidreflector being perforated with a plurality of concentric circulararrays of holes, the size of the holes of the innermost array being thesmallest, the size of the holes of each of the other arraysprogressively increasing toward the edge of said reflector, said feedhaving its radiation pattern directed at the center of said arrays, saidreflector further comprising a plurality of randomly spaced holeslocated between and outside of said circular arrays of holes.

2. The antenna of claim 1 wherein the diameter of the holes of saidinnermost array is approximately equal to the shortest wavelength to beradiated by said antenna and diameter of the holes of the outermostarray is approximately one-half of the longest wavelength to be radiatedby said antenna, the diameters of the holes of the intermediate arraysbeing between those of said inner and outer arrays.

3. The antenna of claim 2 in which said broadbanded horn comprises aridged horn excited by a coaxial line.

4. A broadband, high gain antenna with a relatively constant beamwidthcomprising, a parabolic reflector, a broadbanded feed at the focus ofsaid reflector, the surface of said reflector being perforated with aplurality of concentric circular arrays of holes, the diameter of theholes of the innermost array being approximately one inch, the diameterof the holes of the outermost array being approximately 5.3 inches, thediameter of the circle connecting the centers of the holes of saidoutermost array being approximately 40 inches, the hole diameters of theintermediate arrays being intermediate those of said inner and outerarrays, a hole-free area of approxi mately 6 inches diameter within saidinnermost array, said reflector being further perforated with aplurality of randomly spaced holes between said circular arrays ofholes, and means to apply a microwave signal having a frequency in therange of 1 to 10 gigahertz to said broadbanded feed.

5. The antenna of claim 5 wherein said feed is offset from the apertureof said antenna.

References Cited UNITED STATES PATENTS 2,636,125 4/ 1953 Southworth343-909 X 2,985,880 5/1961 McMillan 343910 HERMAN KARL SAALBACH, PrimaryExaminer T. J. VEZEAU, Assistant Examiner US. Cl. X.R. 343-909, 91 2

